I have, on my way to Odessa, Ukraine back in 2018. I wrote in Illini's Vietnam thread about it.
Definitely go to the Sultan Ahmet complex which is right next to Hagia Sophia. It was constructed between 532 and 537 ADE and was the largest cathedral in the world for almost 1000 years.
But the really freaky place is the Sultan Ahmet complex. Illini mentioned in his thread about the killing fields of Cambodia. You mentioned about visiting Turkey so I posted the following:
in the Sultan Ahmet complex there are mosques filled with coffins. Large coffins, small coffins, really small coffins. In case anyone wondered, when a sultan died, his oldest son would kill all his own brothers and their whole families so as to remove any potential claim for the role of being sultan. So those little coffins were the coffins of the oldest son's brother's children.
Here is what one of the places looks like:
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